When a pair of contacts of a mechanical switch are changed between closed and open condition, it is commonplace for such contacts to transiently undergo a bouncing with each other. Thus a closure, for example, of the contacts is followed by a momentary reopening thereof followed by another momentary closing, and so on, until the transient re-closings and re-openings terminate to leave the contacts stably closed. Conversely, in some switches an initial opening of the contacts is followed by their momentary closure followed by another momentary opening thereof, and so on, until, again the transient reclosings and re-openings terminate to leave the contacts stably open.
The foregoing may be described in mechanical terms as a train of mechanical opening-closing contact impulses and, for some switches, such trains are produced upon switching of the contacts both to closed condition and open condition. The generation of such trains of mechanical impulses creates, however, in switch units for electrically signaling such conditions, the problem that such mechanical impulses are prone to produce in such units corresponding trains of extraneous electrical impulses which may cause the unit to produce a false signal or which, alternatively, may be passed on from the output of the unit to cause spurious or undesirable effects at some point further on.
A technique already used in the art for dealing in switch units with such extraneous electrical impulses is to interpose between the switch contacts and the electrical output of the unit a buffering bi-stable flip-flop responsive to closing and opening of the contacts to indicate the same while concurrently being rendered insensitive to extraneous electrical impulses caused by such closing and opening. Thus, for example, U.S. Pat. No. 4,349,076 issued Sept. 14, 1982 in the name of Christian Oldendorf for "Change-Over Switch with De-Chattering Device for Electronic Weighing Scales" discloses a switch unit comprising a two-contact switch and a bistable semiconductor flip-flop which is responsive to closing and opening of the switch to be triggered to first and second electric states thereof while concurrently not being affected by extraneous electrical impulses produced upon closing of the switch. That switch unit has, however the disadvantages that the flip-flop is required to be of nonstandard asymmetrical design and that, insofar as can be determined, the switch unit includes no provision for dealing with extraneous electrical impulses produced upon opening of the switch.